1. Field of the Invention
This invention relates to thermostat metals and, more particularly, to thermostat metals which have a substantially uniform flexivity over a broad range of useful operating temperatures and which have a rapid response to a change in ambient temperature.
2. Description of the Prior Art
A wide variety of metals are presently available which are made up of two or more layers of metallic alloys bonded to each other. At least one of the layers has a relatively high coefficient of thermal expansion and another layer has a relatively lower coefficient of thermal expansion which causes the thermostat metal to deflect when the ambient temperature is changed. Thermostat metals are used in a variety of different applications to provide electrical and temperature overload protection and to measure or regulate the temperature of a device.
Electrical overload protection may be provided, for example, by a circuit breaker of the type in which a current is conducted through a thermostat metal and the breaker is actuated under overload conditions by the I.sup.2 R loss which heats the thermostat metal. In this type of application, the properties of thermostat metal are comparatively noncritical since, as the breaker is always either fully open or shut, the metal only need have a high flexivity and the desired electrical resistivity to function satisfactorily. Adjusted of the electrical resistivity for a given application is not difficult as a layer of high electrical conductivity metal in a selected thickness can be included as one of the components of the thermostat metal to provide the desired resistivity. U.S. Pat. No. 3,767,370 discloses such a structure in which from 5 to 60% of a copper alloy is used as the electrical conductive element in combination with low conductivity alloys that have relatively low and high coefficients of thermal expansion.
The design of thermostat metals for applications in which the actuating heat is not internally generated presents design problems of somewhat more complexity. For example, in a thermostat used to maintain the temperature of an object at a variably selected temperature, it is important that flexivity be reasonably constant over the operating temperature range of the device -- i.e., deflection should have a linear response to temperature changes for ease in calibration and, if reasonably accurate temperature control is to be maintained, the thermostat metal should reach thermal equilibrium at a rate approximately equal to the rate at which the ambient temperature can change. The latter is true because if the rate at which the thermostat metal reaches equilibrium lags behind the heating or cooling rate, to the extent of such lag, there will be an overshoot in temperature before the thermostat metal responds to temperature changes.
The problem of rapidly reaching thermal equilibrium is essentially absent in the simple circuit breaker discussed above in which current is carried by the thermostat metal. This is so since the thermostat metal is internally heated by the current flowing through it and therefore immediate response is achieved under overload conditions. In distinction to this, however, the thermostat metals with which this invention is primarily concerned are those that must respond to an externally applied heat stimulus. In order for these devices to reach thermal equilibrium, heat must be conducted through the outer layers of the thermostat metal toward the center until a uniform temperature is reached throughout.
In the simple circuit breaker application, both heat and electricity may be conducted along the longitudinal axis of the thermostat metal at a rate substantially limited only by the element having the highest rate of thermal conductivity. In contrast, when the thermal stimulus is external, the limiting factor becomes transverse and thermal conductivity is limited by the elements that have the lowest rate of thermal conductivity. Thus, it does not necessarily follow that a thermostat metal that has a high rate of thermal conductivity along its axis will reach thermal equilibrium rapidly in response to external ambient temperature changes.